Tropical Cool: Hot roof unmitigated

Yesterday we looked at hot tin roofs, today we will see some ways folks try to reduce the heat from same.

First, just to show that I'm not joking about hot roofs, I just took the temp of a piece of roofing which has been sitting in the mild morning sun with a nice cooling breeze for a while. Almost 50 C, as you can see. And guess what, when I check the temperature under the tile, it is within a tenth of a degree of the top temperature. This afternoon the reading will be more than 60 C. (Night time is a different story, and an improtant one from the standpoint of human comfort, but more on that subject later.)

If you were sitting under this roof, and had no hair, you would be suffering, even if the air in the room were cooled, as by an air conditioner or by strong ventilation. This is because the 50 degree underside of the roof radiates about 100 watts per square meter on objects below, regardleess of the air temperature in the room. If the iar temp is say 30, you will feel 40 on your head.

If there is not strong ventilation the air will quickly heat, as much as 5 degrees in a few minutes, and you will suffer from high air temperature as well. With no ventilation you could easily die. (Remember the sorry case of the prisoners loaded into hot trucks?)

Now what a lot of people do is put in some kind of a dropped ceiling, which of course intercepts the radiant heat from the underside of the roof. But even though you can turn off the radiant heat, but the convective transfer will soon enough heat up the air in the "attic", maybe all the way up to the high 40s. Then the gypsum ceiling will heat up, then....you are right back where you started, a hot gypsum ceiling instead of a roof. Sure, there is a little time factor here, each square meter of hot roof has to heat up a couple of cubic meters of air and a little bit of gypsum, but that may be as little as a few minutes, since the hot roof with an R value of less than 1 will deliver about 100 watts per square meter of heat.

But what you can do, is try to get rid of that heat -- 100 watts per square meter over a 5 meter square room is 2500 watts -- but to do this by ventilation you will need about 60 air changes per hour in the room, a change every minute. The requires a pretty good wind moving through the room (or attic). And if there is no wind outside, or you don't have huge openings, and if you don't dela with the radiant heat as well, you are still mighty uncomfortable. (I know, everyone raises their hand and wants to talk wind turbines and fans, but that's for next time.)

Ah, but hasn't the building industry come up with a magic solution, aluminum foil?

More on that, too, next time.

Comments
Ian 12/03/2007 16:47 IP: 124.121.71.249 sgh, my electricity costs are around 1500 baht per month, again like you my Thai friends are horrified, but then I pay 80 pounds per month in the UK on a budget account. My aim is to get it down to 500 baht, not to save money but rather as a technical challenge:-) Sounds like you could easily do a loft conversion, this would also help to keep the groundfloor cooler.	Delete
sgh 12/03/2007 14:07 IP: 124.157.240.105 Ian, thank you for your input it will be helpful. My roof is large and tall ,about 5mtr from top to ceiling of suspended gypsum board, with a pitch of i think 45deg ,with a Lanna style to the front . The tials are a cement pantial and in all a weight of 10 tonnes. This roof sits on a bungalow of 16mtr by 14 mtrs, with AC but no insulation some heat loss from the Lanna part of the roof this has a grill to the front. So you see i have to be environmentally friendly and insulate , my costs are bht 1300to bht 1800 a month and have been acceptable to a farange but the Thais have a fit when they see my bill. Every ones posts have been interesting reading thank you.	Delete
Ian 11/03/2007 13:57 IP: 124.121.71.184 sgh, I have a fan in my loft in England, basically because my internet router lives there and it was overheating and dying:-) At the moment it is on a manual switch, will fit a thermostat soon. Polyurethane foam will certainly work but is expensive, why not glue sheets of expanded polystrene foam to the underside, much cheaper. The main advantage of the polyurethane foam is it is fire resistant, but both foams give off heavy highly toxic fumes when heated so pushing the urethane is a sales gimmick rather than a safety one. But it is still better to reflect the heat away rather than block its conduction. If your roof overheats it may expand enough to create cracks and stop being waterproof.	Delete
sgh 11/03/2007 13:10 IP: 124.157.240.210 i have seen in American movies ,an electric fan built in to the gable end of the roof this may have a stat on it . A practice i have not seen in England but with us getting hotter may happen.	Delete
sgh 11/03/2007 13:03 IP: 124.157.240.210 should be polyurethane sorry	Delete
sgh Phetchaburi. 11/03/2007 13:00 IP: 124.157.240.210 Does anyone know if polyuethane rigid foam sprayed on to the underside of a roof is good or not, LOHR TRADE & CONSULTING PTS,LTD http://www.1stplanet.com/lohr-trade advert in paper ,seems a good way ,i would do it if it worked.	Delete
Ron Swelters 09/03/2007 17:17 IP: 124.120.225.72 Some more interesting ideas I hadn't thought of, I'll be discussing these again when we get to preventive measures.	Delete
Ian 09/03/2007 10:04 IP: 124.121.72.45 Ron, if you buy a small pump and spray "atomised" water on your roof, the evaporative cooling obtained is very good. A small pump and a few litres per hour of water is much cheaper than the equivalent aircon unit. ........Energy watch, these high efficiency bulbs are miniature fluorescent lights, basically a Ultra-Violet discharge exciting a phosphor coating, there is almost zero Infra-Red radiation from them, and the visible light is at the high end of the spectrum. As to other lights, I decorated my main room with lots of twinkling coloured lights for Christmas, my partner liked this so much I cannot take them down now:-)	Delete
EnergyWatch 09/03/2007 08:23 IP: 58.136.225.121 Ron,,, I suggest an idea how to cool your burning tin roof.>>> You can find around you, the weaved long grass that is used to rice pad cottage usually. A piece costs 10-15 baht. You can lay them over the tin root. Also you can spray the water over the grass at sizzling sun light. It will not only cool the tin but also absorbs heat around by the water vapours.	Delete
EnergyWatch 09/03/2007 06:06 IP: 58.136.227.253 Ian, from the departure of Plank's, I try to imagine if your 15w light bulb could send unpleasant frequency (of the light) to your Thai partner. I also brought why candle light makes most of women feel romantic, apart from your knowledge of chicken. This is my method to find new intelligence, HaHaHa >>> Well, why dont you change the bulb, the one immitating candle flame. I have one, a string for christmas tree and I found my daughter likes it more than other brightly flinkering ones. Ian 08/03/2007 20:12 IP: 124.121.73.153 Energy watch, I think I understand what you mean, even though your physics is very strange:-) I think you are quoting Plank's constant E=hν, where E is energy, h is Planck's constant, and ν (Greek letter nu) is frequency. Yes artificial light will stimulate a chicken to lay eggs, so it probable has a greater effect on a woman than a man:-) In effect you are saying that artificial light increases a woman's resting state metabolism, an interesting suggestion. Delete EnergyWatch 08/03/2007 17:59 IP: 58.136.227.134 Ian,,,Energy of photon is measured by its frequency, not density (here, like Lux) but certainly the energy may afflict her bio-rhythme assuming photom provokes electrons on her nerve and her body engine burns more glucose as if phonton excites electrons in photovoltaic semiconductor (Quantum theory). So not all for her psychology. >>> Most of women want to turn off light or reduce into wax candle size in night time. I suspect her reproduction system relates !. Delete Ian 08/03/2007 13:23 IP: 124.121.73.151 energy watch, now I know where my Thai companion gets her strange ideas, that a 15w high efficiency light bulb makes her feel hot, it is part of her cultural psychology:-) Remove the sun's radiation by being in a dark room, turning on a 15w bulb will make no difference to the heating, the output of the bulb is primarily in the visible part of the spectrum not the infra red. ............with ceiling height there is a conflict, a high ceiling will allow the hotter air to rise, but a low ceiling reduces room volume and makes life easier for the aircon. EnergyWatch 08/03/2007 12:46 IP: 58.136.227.210 In Attic, I installed two fans, suction and exhaustion to keep gymsum board cooled when no proper wind moves the hot chamber airholding.>>> Solar Light is made of photon, the elementary particles of having frequency and wavelength. This particles bombards other materials (like air, oxygen and nitrogen) to generate heat. From this theory, If you keep your room in dark, it will be cooler. The solar light does not carry heat but when it bombards your skin, the skin generates heat to warm your body.>>> In daytime, I come down to ground floor room that has good granite floor, darkened by thick branches of kapoke tree, facing my fish pond that disperse sun light away from my room and keep fine humidity around, all hot air is guided to flow up through the stairway corridor upto attic exhausting fan or naturally, It is cool to work on this computer. At two - three PM the hottest time zone, I showered at attached bathroom and lie down on granite floor and take a nap.>>> If one tries, there are good construction materials, such as heat preventing hollow brisk, double brick with inner foam filler or roof padding insulation foam or fabrics. In design, there are many wisdom to prevent sunlight and warm air invasions. One of them is concrete wings that is found in many college or business buildings. This wing-like fin prevents sunlight invasion as well as expelling the heat of building structure. The structure by itself is designed for natural ventilation, generally vertical corridor through which all interior warmed air is expelled and refill !!!. Such house has generally lofty roofing structure like Temple. Go and sit in large mass hall in a temple, It is cool because of lofty ceiling and wise ventilation arrangement.	Delete